1. Field of the Invention
The present invention relates to an apparatus for adjusting the light of a lamp, such as one serving as a light source for illuminating documents inserted in a copying machine.
2. Description of the Prior Art
Halogen lamps are chiefly used for copying machines as light sources for illuminating documents. The light of the halogen lamp is adjusted by applying a commercial a.c. power supply as controlled in phase. FIG. 6 shows an example of circuit conventionally used as such a light adjusting apparatus. As shown in the drawing, a lamp 1 is connected to a commercial a.c. power supply AC through a triac T serving as a switching element. The triac T is controlled in phase in accordance with an output signal from a switching control circuit 3 via a pulse transformer PT. The current from the power supply AC is directly rectified by a diode bridge DB to a direct current, which operates the switching control circuit 3. The circuit diagram further shows a coil L serving as a rush current restricting reactor, a gate current rectifying diode D1 for the triac T, and a diode D2 for absorbing a spike due to the pulse transformer PT.
Thus, the switching control circuit 3 is driven by the direct current obtained by directly rectifying the current of the commercial a.c. power supply without using any isolating transformer to render the overall circuit less costly.
FIG. 7 shows the circuit of another conventional light adjusting apparatus. This circuit differs from the circuit of FIG. 6 in that the pulse signal to be produced by a switching circuit 3 is controlled in phase by a light adjusting signal generating circuit 2. When the control circuit 3 is thus given a light adjusting signal from outside, the power supply for the circuit 3 must be isolated from the commercial a.c. power supply. For this purpose, a power supply equipped with an isolating transformer 4 is used for the control circuit 3 as illustrated.
However, these conventional light adjusting apparatus have problems. When the current obtained by directly rectifying the commercial a.c. power supply is used for the switching control circuit as shown in FIG. 6, the circuit of the apparatus can be simplified in construction and is available at a reduced cost, but the light must be adjusted as by a variable resistor VR, while it is impossible to use an external light adjusting signal for phase control. Further when the switching control circuit is driven by the insulated power supply as seen in FIG. 7, it is possible to give a light adjusting signal from outside directly to the control circuit, whereas there arises a need to use the insulating transformer therefor, rendering the apparatus costly and large-sized.